The present invention relates to a method for producing printing cylinders for seamless or endless patterns with printing form engraving machines having scanning and engraving cylinders. A pattern original on the scan cylinder is opto-electronically scanned picture-element-wise along scan lines positioned next to one another which proceed in a circumferential direction of the scan cylinder, and wherein every scan line supplies the information for an engraving line of a pattern repeat to be transferred to the engraving cylinder.
Printing form engraving machines are in use in printing technology with which printing forms are engraved for printing seamless and endless patterns. An example of such endless patterns is a wood decoration for the manufacture of which a printing form comprising an endless pattern is employed. Pattern originals which are produced, for example, by photographing a real wood pattern and by subsequent retouch are employed for the production of such patterns. A difficulty in the reproduction of the pattern is to make the seam between beginning and end of the pattern disappear, because a joint is immediately perceptible to the eye in the endless pattern. For this purpose, the original is retouched at a beginning and at an end so that an optimally uniform transition between the decorative path at the beginning and end of the pattern or of the original should occur. Nonetheless, the seam in the final printed product is usually still perceptible in this technique. For the suppression of remaining residual seams, for example, it is known from the German Pat. No. 16 52 340 and from the German OS No. 31 29 649, both incorporated herein, to produce the pattern original such that it is longer than the pattern repeat, whereby the length of the basic pattern is referred to as pattern repeat.
In the German Pat. No. 16 52 340, the change-over location, i.e. the joint, is displaced into the pattern beginning and into the pattern extension. An improvement thus arises in comparison to the simple case wherein pattern beginning and end follow one another, and the transition at the joint is to be avoided only by means of retouching the pattern beginning and end. In many cases, however, the seam nonetheless remains perceptible since, for example, wood grows irregularly as a natural product and the grains at the pattern beginning and end often deviate too greatly from one another for this deviation to be completely eliminated by retouching.
In the German OS No. 31 29 649, the image signals in the original scan which are acquired by scanning that part of the original proceeding beyond the pattern repeat are mixed with the image signals acquired by the scan of the beginning of the original. They are mixed in a mixing region by a computer for generating a steady transition between the signal level at the beginning of the pattern repeat and the signal level at the end of the pattern repeat. Although a steady transition occurs here, since the mixing region always begins at the beginning of the original and has a prescribed length, a strip nonetheless arises in the pattern printed with such a printing form. This strip deviates from the remaining pattern in terms of its pattern structure and is perceived by the eye as a disturbance repeating after every repeat. These individual techniques which have been proposed in fact yield improvements in the pattern matching but the result in the individual methods is not satisfactory.